The present invention relates to a device for incorporation in an apparatus for automatic wire bonding, in particular for wedge bonding using thick wire having a diameter of at least 4 mil, which device makes it possible to monitor the amount of wire consumed during wire bonding, and to monitor the wire for breakages or feeding failures.
Wire bonding is the process of making electrical connections in semiconductor components by means of metal wire. Examples of electrical connections which can be made using wire bonding techniques include connections between the contact surfaces of discrete or integrated chips and the contact leads of their packages, and, in the case of hybrid circuits, the connections between inserted monolithic elements and the film circuit which contains them.
A number of wire bonding techniques have been developed, and one which has been particularly successful is a microwelding technique using ultrasound. An automatic wire bonding apparatus on which such a technique can be operated is described in German Patent Application No. P 33 43 738. Aluminum wire, in contact with the contact surface to which it is to be bonded, is moved vigorously in the direction of the surface to which it is to be bonded so that its oxide layer breaks open. The wire is then subjected to pressure, and a permanent junction is created between the two materials. Motion of the wire is generated by an ultrasonic transducer excited by an ultrasonic generator to produce high-frequency mechanical vibrations.
In the particular wire bonding process known as wedge bonding, the ultrasonic energy is supplied in the range of 1 to 50 watts, depending on the wire size used. The ultrasonic energy is directed to the aluminum wire by a special tool known as a "wedge". The wire is fed through a hole at the bottom of the wedge. When the wedge with the aluminum wire touches the surface to which the wire is to be bonded, movement is stopped. The wire is pressed down with a small defined force, known as the bonding weight, and the wire is slightly deformed. This small deformation is known as the "pre-deformation". Ultrasonic energy is now switched on, and the welding process starts. During this time, the diameter of the aluminum wire is reduced by a few microns, the actual reduction depending on the size, physical properties and the precise chemical nature of the wire.
In known wire bonding apparatus, the wire is fed directly from a wire feed spool to the bonding wedge and the amount of wire consumed during a bonding operation is not monitored.
It is important, particularly in an automatic wire bonding apparatus, for the operator to have as much control over the process as possible. In order to achieve this, it is important for the operator to have as much information as possible on the progress of the bonding operation. In this connection, it would be helpful to the operator to be able to monitor the amount of wire consumed, and also to monitor the wire to ensure that if the wire breaks or the feed jams, this can be remedied as soon as possible.
It is a disadvantage of known wire bonding apparatus that there is no provision in such apparatus for monitoring the amount of wire consumed, or whether there is any breakage or feeding difficulty with the wire.
It is an object of the present invention to provide a device for incorporation in an apparatus for wire bonding, in which the above disadvantages are reduced or substantially obviated.